This invention pertains to the measurement of torque on rotating machinery and more particularly to apparatus for measuring the air gap torque acting on the rotor of an electrical generator.
Electrical power generators such as those in common use by the electric utility companies utilize a rotating magnetic field created by the field winding carried by the generator rotor. The magnetic field interacts with the stator armature winding in a well known manner to produce voltage and current which may be supplied to a load. Virtually all generators of this kind are 3-phase machines in which the output quantities are substantially equally separated in phase angle.
As the rotor turns, a torque is produced on the rotor, acting through the air gap that exists between the rotor and the stator, that results from the electromagnetic field interaction with the stator winding. This torque, frequently referred to as air gap torque, is proportional to the electrical power produced by the generator and inversely proportional to the rotor angular velocity.
The generator, of course, is designed to withstand all normal torque conditions and to have appropriate safety margins so that even short duration, transient torques of extreme magnitude do no immediate permanent damage to the machine. Very high levels of air gap torque can result, for example, from line faults such as line to ground or line-to-line short circuits on the power lines connected to the generator output terminals. The effects of these very high levels of torque are cumulative, however, so that over a long period of time in which a generator has sustained a number of such incidents, structural fatigue damage may ultimately be produced which forces the machine out of service.
It is an object of the present invention, therefore, to provide apparatus for continuously monitoring the resulting air gap torque on the rotor of a 3-phase generator by sensing certain of the output voltages and currents produced by the generator as it supplies power to a connected load. By these means it becomes possible to quantify the air gap torque under both normal and transient conditions and to use such torque information for predicting future machine outages and for deriving certain other information regarding past incidents of power line faults.
In a preferred form of the invention, signals representing the three generator output voltages and two of the three line currents are conditioned and combined in electrical circuitry to produce a single output signal which is indicative of the air gap torque acting on the generator rotor. The torque signal so produced is in a form easily handled by conventional means such as computers and/or recorders, and is appropriate for use by operating personnel in making decisions regarding generator operation.